1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transmitters and/or receivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read and/or write terminal. Generally, transponders extract the power supply required by the electronic circuits included therein from the high-frequency field radiated by an antenna of the read-write terminal. The present invention more specifically relates to a read and/or write terminal for electromagnetic transponders as well as to the antenna that it includes.
2. Discussion of the Related Art
FIG. 1 very schematically shows a conventional example of a read/write terminal 1 associated with a transponder 10.
Generally, terminal 1 is essentially formed of a series oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1. This oscillating circuit is controlled by a device 2 including, among others, an amplifier or antenna coupler and a control circuit exploiting the received data provided, in particular, with a modulator/demodulator and a microprocessor for processing the control signals and the data. The oscillating circuit is excited by a voltage Vg provided by device 2 between terminals 3 and 4. Circuit 2 generally communicates with different input/output circuits (keyboard, screen, means of exchange with a server, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal draw the power necessary to their operation from a supply circuit (not shown) connected, for example, to the electric supply system.
A transponder 10, intended for cooperating with a terminal 1, essentially includes a parallel oscillating circuit. This circuit is formed of an inductance L2 in parallel with a capacitor C2 between two input terminals 11, 12 of a control and processing circuit 13. Terminals 11, 12 are in practice connected to the input of a rectifying means (not shown), the outputs of which form D.C. supply terminals of the circuits internal to transponder 10. These circuits generally include, essentially, a microprocessor, a memory, a demodulator of the signals that may be received from terminal 1, and a modulator for transmitting information to the terminal.
The oscillating circuits of the terminal and of the transponder are generally tuned on a same frequency corresponding to the frequency of excitation signal Vg of the terminal's oscillating circuit. This high-frequency signal (for example, at 13.56 MHz) is not only used as a carrier of data transmission from the terminal to the transponder, but also as a remote supply carrier for the transponders located in the terminal's field. When a transponder 10 is located in the field of a terminal 1, a high-frequency voltage is generated across terminals 11 and 12 of the transponder's resonant circuit. This voltage, after being rectified and possibly clipped, provides the supply voltage of electronic circuits 13 of the transponder.
The high-frequency carrier transmitted by the terminal is generally modulated in amplitude by said terminal according to different coding techniques to transmit data and/or control signals to one or several transponders in the field. In return, the data transmission from the transponder to a terminal is generally performed by modulating the load formed by resonant circuit L2, C2. The load variation is performed at the rate of a sub-carrier having a frequency (for example, 847.5 kHz) smaller than that of the carrier. This load variation can then be detected by the terminal as an amplitude variation or as a phase variation by means, for example, of a measurement of the voltage across capacitor C1 or of current Ig in the oscillating circuit. In FIG. 1, the measurement signal has been symbolized by a connection 5 in dotted lines connecting the midpoint of the series connection of inductance L1 and capacitor C1 to circuit 2.
A problem which arises in conventional transponder systems is that they generally have a limited range. The system range corresponds to the limiting distance beyond which the field sensed by a transponder is too small to enable extraction of the power necessary for its operation therefrom. The limited range is essentially due to the maximum admissible magnetic field, which is set by standards. Conventionally, to increase the range, the diameter of the antenna is desired to be increased, to avoid exceeding this maximum allowed magnetic field. Now, increasing the diameter amounts to increasing excitation current Ig in proportions that are not desirable, among others, for power consumption reasons.